The following description provides a summary, of information, and is not an admission that any of the information provided or publications referenced herein is prior art to the present disclosure.
Platelet-derived growth factors (PDGF-A, -B, -C and -D) are ubiquitous mitogens and chemotactic factors for many connective tissue cells (Fredriksson, L., et al. (2004) Cytokine Growth Factor Rev. 15(4):197). PDGFs occur as disulfide-linked dimers and contain a cysteine-knot-fold growth factor domain that functions through binding to PDGF receptors α and β on the surface of a cell (Claesson-Welsh, L., et al. (1989) Proc. Natl. Acad. Sci. USA 86:4917). PDGF binding induces receptor dimerization, which leads to autophosphorylation at intracellular tyrosine residues (Claesson-Welsh, J. (1994) Biol. Chem. 269:32023). PDGF-BB is involved in several proliferative disorders, including atherosclerosis, fibrosis, macular degeneration, and cancer (Östman, A., et al. (2001) Adv. Cancer Res. 80:1; Appelmann, I., et al. (2010) Recent Results Cancer Res. 180:51; Trojanowska, M., et al. (2008) Rheumatology (Oxford) 47(Suppl 5):2; Rutherford et al. (1997) Atherosclerosis 130:45; Smits et al. (1992) Am. J. Pathol. 140:639; Heldin et al. (1991) Endocrinology 129:2187; Floege and Johnson (1995) Miner. Electrolyte Metab. 21:271; Raines et al. (1990) Experimental Pharmacology, Peptide Growth Factors and Their Receptors, Sporn & Roberts, pp. 173-262, Springer, Heidelberg).
VEGF is a secreted disulfide-linked homodimer that selectively stimulates endothelial cells to proliferate, migrate and produce matrix-degrading enzymes, all of which are processes required for the formation of new blood vessels (Conn, G., et al. (1990) Proc. Natl. Acad. Sci. USA 87:1323; Ferrara, N. et al. (1989) Biochem. Biophys. Res. Commun. 161:851; Gospodarowicz, D., et al. (1989) Proc. Natl. Acad. Sci. USA 86:7311; Pepper, M. S., et al. (1991) Biochem. Biophys. Res. Commun. 181:902; Unemori, E. N., et al. (1992) J. Cell. Physiol. 153:557). In addition to being the only known endothelial cell-specific mitogen, VEGF is unique among angiogenic growth factors in its ability to induce a transient increase in blood vessel permeability to macromolecules (Dvorak, H. F., et al. (1979) J. Immunol. 122:166; Senger, D. R., et al. (1983) Science 219:983; Senger, D. R., et al. (1986) Cancer Res. 46:5629). Increased vascular permeability and the resulting deposition of plasma proteins in the extravascular space facilitate new vessel formation by providing a provisional matrix for the migration of endothelial cells. Hyperpermeability is indeed a characteristic feature of new vessels (Dvorak, H. F., et al. (1995) Am. J. Pathol. 146:1029). Furthermore, compensatory angiogenesis induced by tissue hypoxia is also mediated by VEGF (Levy, A. P., et al. (1996) J. Biol. Chem. 271:2746; Shweiki, D., et al. (1991) Nature 359:843). The identification of VEGF as a hypoxia-inducible protein, along with the complementary observation that hyperoxia causes suppression of VEGF expression, provides an appealing mechanism for matching oxygen demand with vascular supply (Benjamin, L. E., et al. (1999) J. Clin. Invest. 103:159; Alon, T., et al. (1995) Nat. Med. 1:1024).
Several isoforms of VEGF protein occur as a result of alternative splicing of the eight exons of the gene that encodes VEGF (Eming, S. A., et al. (2006) J. Invest. Dermatol. Symp. Proc. 11:79). The most prevalent isoforms are VEGF-121, VEGF-165 and VEGF-189. Proteolytic processing of VEGF can generate additional isoforms. VEGF-165 can be cleaved by plasmin between Arg-110 and Ala-111 to generate VEGF-110, which is functionally equivalent to VEGF-121 (Keyt, B. A., et al. (1996) J. Biol. Chem. 271:7788). VEGF-189 can be cleaved by urokinase within the exon 6 domain and then can be cleaved further by plasmin to generate VEGF-110 (Plouët, J., et al., (1997) J. Biol. Chem. 272:13390). In addition, a subset of matrix metalloproteases (MMPs), including MMP-3, -7, -9 and -19, are capable of cleaving VEGF-165 and VEGF-189 in sequential steps to generate VEGF-113, which is functionally equivalent to VEGF-110. Therefore, the relative abundance of matrix-bound and diffusible forms of VEGF in a given tissue is determined by the combination of alternative splicing and proteolytic processing that occurs in the cells of the tissue (Ferrara, N., et al. (2006) Retina 26:859).
Age-related macular degeneration (AMD) remains the leading cause of blindness in people over 55 years of age. The disease is characterized by the formation of insoluble deposits called drusen within the macula, the part of the retina that has the highest density of photoreceptors and is involved in central vision. In the initial stages of AMD, the deposits are avascular and the disease generally progresses slowly. However, in about 10% of the patients, this so-called “dry” form of AMD becomes vascularized and turns into the “wet” form of AMD, during which the disease becomes more progressive and vision deteriorates at a faster rate. In many cases, the progression from blurriness of central vision to virtual blindness occurs in less than two years. In the advanced stage of the disease, the exudative or wet form of AMD, new blood vessels penetrate from the choriocapillaris into the central part of the retina (macula), occluding central vision. In the United States, the prevalence of wet AMD is about 1.8 million and is expected to increase to close to 3 million by 2020. The incidence of wet AMD in the United States is about 210,000 people each year.
Recently, AMD has been treated by blocking VEGF-mediated induction of angiogenesis and blood vessels leakiness by direct injection into the eye of high-affinity antagonists that bind to VEGF, preventing interaction of VEGF with its cell-surface receptors on endothelial cells.
There is considerable evidence that dual inhibition of VEGF and PDGF-B signaling leads to more efficient blocking of angiogenesis coupled with regression of new blood vessels. For example, clinical evidence suggests that dual inhibition of VEGF and PDGF-B can achieve a more complete inhibition of ocular angiogenesis in AMD patients. An aptamer inhibitor of PDGF-B (E10030), originally discovered at NeXstar Pharmaceuticals (Green, L. S., et al. (1996) Biochemistry 35:14413; U.S. Pat. Nos. 6,207,816; 5,731,144; 5,731,424; and 6,124,449), is being developed by Ophthotech Corporation as a treatment for AMD. E10030 (Fovista®) is a DNA-based modified aptamer that binds to PDGF-AB or PDGF-BB with a Kd of approximately 100 pM and inhibits the functions of PDGF-B both in vitro and in vivo.
In a Phase 1 study, anti-PDGF therapy with E10030 tested in combination with Lucentis® anti-VEGF therapy resulted in vision gain of three lines in 59% of treated patients after 12 weeks of therapy. This is a considerably higher percentage of patients with improved visual acuity compared to the 34-40% observed historically with Lucentis alone. In addition, the combination treatment was accompanied with marked neovascular regression in all study participants. Enhanced efficacy with combination treatment was recently corroborated in a phase 2 study of 449 patients with wet AMD. Patients receiving the combination of Fovista (1.5 mg) and Lucentis gained a mean of 10.6 letters of vision at 24 weeks, compared to 6.5 letters for patients receiving Lucentis monotherapy (p=0.019), representing a 62% additional visual acuity benefit.